1. Field of the Invention
The invention relates to an abnormality detection apparatus and an abnormality detection method for an air/fuel ratio sensor.
2. Description of the Related Art
An internal combustion engine for a motor vehicle or the like is provided with an air/fuel ratio sensor that outputs a signal that corresponds to the air/fuel ratio of the internal combustion engine on the basis of the oxygen concentration in exhaust gas. On the basis of the output from the air/fuel ratio sensor, the amount of fuel injected into the engine is corrected to so that the air/fuel ratio of the engine becomes equal to a stoichiometric air/fuel ratio. By controlling the air/fuel ratio of the internal combustion engine to the stoichiometric air/fuel ratio through the correction of the amount of fuel injection, good performance of exhaust purification of an exhaust purification catalyst provided in an exhaust system of the engine is maintained so that the exhaust emission of the internal combustion engine can be bettered.
With the foregoing internal combustion engine, there is a risk that abnormality of the air/fuel ratio sensor, such as degradation thereof or the like, may influence the exhaust emission. Therefore, in order to prevent such influence, the engine is provided with an abnormality detection apparatus that determines the presence/absence of abnormality of the air/fuel ratio sensor. A known abnormality detection apparatus of this kind determines the presence/absence of abnormality of an air/fuel ratio sensor by the following procedure “1” to “3” as shown in, for example, Japanese Patent Application Publication No. 2008-14234 (JP-A-2008-14234). Firstly, as the process “1” in the procedure, an active air/fuel ratio control in which the air/fuel ratio of the internal combustion engine is periodically fluctuated between a rich state and a lean state is performed. Next, as the process “2”, a parameter that corresponds to the responsiveness of the output of the air/fuel ratio sensor is found on the basis of the output of the sensor during the active air/fuel ratio control, and the parameter is acquired as data for detecting abnormality. Finally, as the process “3”, the presence/absence of abnormality of the air/fuel ratio sensor is determined on the basis of comparison between the acquired data and an abnormality criterion value.
Incidentally, with regard to the processes “2” and “3”, in order to more accurately perform the determination as to the presence/absence of abnormality of the air/fuel ratio sensor, it is also possible to adopt a modification in which the foregoing acquisition of data is performed a plurality of times, and the presence/absence of abnormality of the air/fuel ratio sensor is determined on the basis of comparison between an average value of the data acquired by the plurality of acquisitions and an abnormality criterion value.
By the way, in recent years, the requirement for betterment of exhaust emission of the internal combustion engine has become severer. If an air/fuel ratio sensor that does not meet the requirement is considered as being abnormal, the responsiveness parameter found in the process “2” represents less clearly a difference made by the presence/absence of abnormality of the air/fuel ratio sensor. In particular, during the state of small amount of intake air of the internal combustion engine, since the exhaust gas pressure of the internal combustion engine (that corresponds to the amount of flow of exhaust gas) becomes low so that the influence caused by abnormality of the air/fuel ratio sensor, such as degradation thereof or the like, does not clearly appear in the output of the air/fuel ratio sensor, the foregoing tendency of the responsiveness parameter representing less clearly the difference made by the presence/absence of abnormality of the air/fuel ratio sensor becomes conspicuous. Furthermore, when the motor vehicle is accelerating or decelerating during the small-amount-of-intake-air state of the internal combustion engine, the responsiveness parameter greatly fluctuates due to the response delay of various appliances of the internal combustion engine, so that there is high possibility that data acquired in the process “2” will have a value that makes it hard to determine the presence/absence of abnormality of the air/fuel ratio sensor.
Therefore, JP-A-2008-14234 proposes that a condition that the internal combustion engine be in a state of large amount of intake air is set as an execution condition for performing the determination as to the presence/absence of abnormality regarding the output of an air/fuel ratio sensor. During the large-amount-of-intake-air state of the internal combustion engine, the exhaust gas pressure of the internal combustion engine (corresponding to the amount of flow of exhaust gas) rises, and influence of abnormality of the air/fuel ratio sensor, such as degradation thereof, or the like, appears clearly in the output of the air/fuel ratio sensor, so that the responsiveness parameter found in the process “2” is more likely to represent a difference made by the presence/absence of abnormality of the air/fuel ratio sensor. Therefore, if the condition that the internal combustion engine be in the large-amount-of-intake-air state as an execution condition for performing the determination as to the presence/absence of abnormality of the air/fuel ratio sensor on the basis of the procedure “1” to “3”, it becomes possible to accurately perform the determination as to the presence/absence of abnormality.
By setting the condition that the internal combustion engine be in the large-amount-of-intake-air state as an execution condition for performing the determination as to the presence/absence of abnormality of the air/fuel ratio sensor, it becomes possible to accurately perform the determination as to the presence/absence of abnormality of the air/fuel ratio sensor. However, it is inevitable that the opportunities of executing the determination as to the presence/absence of abnormality of the air/fuel ratio sensor on the basis of the procedure “1” to “3” decrease by an amount that corresponds to the setting of that condition.